A contactless smart card is commonly used for security access or payment systems. Contactless smart cards generally include an antenna, in the form of an inductor, coupled to an integrated circuit (IC). The IC commonly includes a capacitor which forms a resonant circuit with the antenna. A card reader presents an alternating magnetic field that excites the inductor/capacitor resonant circuit, which in turn energizes and powers the IC. The IC can then perform one or more functions, such as transmitting a card number through the antenna to the card reader.
Some smart cards require supply voltages to power both digital (e.g., 1.4 volts) and analog circuits (e.g., 3.8 volts). The supply voltages are generated from the voltage across the capacitor in the resonant circuit. To ensure stable Direct Current (DC) supply voltages, the alternating voltage is rectified and regulated using, for example, a bridge rectifier and regulator circuits.
Some conventional smart cards use two regulators. A first, shunt, regulator provides loose regulation (e.g., +/−1 volt swing) of an analog supply voltage. The loose regulation allows for reception/transmission by field modulation. A second regulator, for powering digital circuitry, is powered from the analog supply voltage provided by the first regulator.
These conventional smart cards suffer from processor noise (being seen by the card reader) which can cause various problems with reception by the reader of data from the smart card. Moreover, the first regulator often requires the use of a large decoupling capacitor to support the second regulator. A large decoupling capacitor requires a large area and can attenuate modulation at low field strengths during transmission and reception.